Tumors result from an imbalance between cell proliferation and cell death. In addition to mutations that affect cell proliferation, tumors select for genetic alterations that inhibit apoptosis or programmed cell death, especially in later stages of tumor progression. Thus, most malignant tumors acquire an increased threshold for apoptosis that contributes to tumor size and to the resistance of tumor cells to treatment. Despite our increased understanding of the mechanisms that control apoptosis, very little is known about the signals that suppress apoptosis in tumor cells. We have identified a new intracellular signaling molecule, MRK (MLK and MEKK related kinase), and found that it contributes to the suppression of apoptosis. We showed that a kinase inactive MRK interferes with the ability of oncogenic H- rasV12 to transform cells and that an active form of MRK protects IL-3 dependent cells from cell death induced by survival factor removal. In addition, we have established that MRK interacts with HAX-1, an integral membrane protein localized largely to mitochondria and bearing significant homology with the Bcl-2 family of proteins that are known to regulate apoptosis. Our general hypothesis is that abnormal cell survival mechanisms are important determinants of tumor growth and that by interfering with deregulated survival signals, one might increase the susceptibility of tumors to apoptosis and thus improve cancer therapy. The focus of the current proposal is to test the hypothesis that the novel kinase MRK mediates cell survival signals that contribute to the suppression of apoptosis in cancer. Studies in Aim 1 will elucidate the role of abnormal MRK activity in the suppression of apoptosis in cancer cells challenged with chemotherapeutic agents. Studies in Aim 2 will characterize the interaction between MRK and its binding partner HAX-1, and determine the role of this interaction in the MRK anti-apoptotic function. Studies in Aim 3 will address the mechanism of suppression of cell death by MRK by testing its effect on the regulation of known pro-apoptotic proteins, such as BAD and caspase 9. Our long-term objectives are to elucidate the signal transduction pathway mediated by MRK and to understand the mechanisms that contribute to the increased resistance of cancer cells to induction of apoptosis.